Because the rivers don't go exactly through our fields, and not every field has irrigation built in (in fact around where I am I'd say 95% DON'T) so yes the rainfall is important.

And get this: the locks and dams on the Mississippi river north of southern MN are CLOSED, meaning no barges or river travel is allowed on the river because the river is TOO SHALLOW for the barges to get through. The barges need a certain depth and the river is too small to get through now. Yes, rainfall is important.